1. Field of the Invention
The invention relates to a tuning circuit for a receiver, in which a local oscillator, having a voltage-controlled frequency, is constituted by an amplifier which is coupled to a resonant circuit of the variable capacitance and inductance type, an output terminal of the amplifier being positively fed back to an input of the amplifier and to a terminal of the resonant circuit, in which circuit the amplifier has a first stage formed by a differential pair of bipolar transistors whose linked emitters are connected to a current source, which pair is formed by a first transistor whose base is connected to an input terminal of the amplifier and a second transistor whose collector is coupled to a main load resistor having one end connected to a power supply terminal and the other end connected to an output terminal of the amplifier.
The invention also relates to an integrated circuit for a receiver comprising elements of the tuning circuit mentioned above, as well as to a television receiver in which such a tuning circuit is used.
2. Description of the Related Art
A tuning circuit of the type mentioned above is present in practically all radio or television receivers in which it is used for user selection of one of the available channels. In this tuning circuit, the local oscillator plays a very important role for producing an intermediate frequency signal by means of a frequency difference between the signal to be received and the signal of this local oscillator. Particularly in television receivers, it is often difficult to obtain an oscillator which is capable of functioning in a wide range of high frequencies.
As is known, the local oscillator of a receiver is constituted by an amplifier device coupled to a resonant circuit most frequently constituted by a capacitance-inductance combination. It is nowadays preferred to construct this amplifier from a differential pair of transistors instead of using a single transistor. Better performances are obtained particularly as regards the absence of even harmonics in the signal produced by the local oscillator, and due to the fact that the input impedance of the amplifier, which remains high for the greater part of a period of the output signal, is thus high on average.
An oscillator activated by means of a differential pair of transistors is known from the document EP-A-0 412, corresponding to U.S. Pat. No. 5,034,706, in which a resonant circuit of the LC type is indicated.
When it is attempted to extend the range of frequencies covered by an LC resonant circuit oscillator in which the variable element is a variable capacity diode as a function of a control voltage, it will be experienced that there is a considerable limitation at the lowest frequencies where the oscillator ceases to function. This phenomenon is related to a resonant circuit coefficient of insufficient quality, which is due to high losses in the variable capacity diode.
As the variable capacitor C is arranged in parallel with an inductance L, the equivalent parallel loss resistance Rp of this capacitor may be evaluated by means of a simple computation based on the series loss resistance R.sub.s resulting in: EQU Rp=L/(R.sub.s .multidot.C)
It will be noted that the parallel loss resistance Rp decreases when R.sub.s and/or C increase and that an oscillation is thus difficult at the low frequencies in the wanted range, under which circumstances, the capacitor has a high value.
A first approach to solving this difficulty may be the use of several series-parallel diodes for obtaining a variable capacitance of a better quality.
However, this solution is hardly satisfactory because the losses are only reduced to a small extent as a function of the number of diodes used, which solution would be costly and cumbersome if a very large number of diodes had to be used.
A second approach which is frequently used is to arrange a fixed capacitance (substantially free from losses) in series with the variable capacitor, which fixed capacitance is known as a "padder". In accordance with this solution, the losses are effectively reduced but the bandwidth of the frequencies covered is also reduced, which is contrary to the envisaged object.